The properties of a material are influenced by the surrounding environment and can even vary with its particle size. The surface area of a material will change with its particle size. The properties such as magnetism, electric resistance and optical properties of a material with a smaller particle size will be different from those of the same material with a larger particle size due to the relative difference between their surface area.
It is known that ultra-fine powders of metal oxides are useful in various fields. For example, for catalysts, the application of ultra-fine metal oxide powders can increase the surface area of catalyst; for computers and televisions, the application of ultra-fine metal oxide powders in coating the screen of a computer or a television can provide resistance to electrostatics and reflection; and in chemical mechanical polishing, ultra-fine metal oxide powders can be used as abrasives in the polishing procedure. Various processes for the preparation of metal oxide powders are known in the art. For instance, cracking, physical vapor deposition, chemical vapor deposition, spray pyrolysis, gel method and hydrothermal method have been developed (see, Carturan et al., J. Material Science 23 (1988) 3156; Xie et al., Phys. Stat. Sol. (a) 141, K59 (1994); Unaogu et al., Solar Energy Materials 20 (1990) 29; J. P. Chatelon, Terrier et al., Thin Solid Films, 247 (1994) 162 and Zuo et al., J. Appl. Phys. 75(3), 1 February 1994.) Cracking is simple but cannot provide the desired particle size and distribution because the particles prepared are not uniform. Both physical and chemical vapor depositions have to be conducted under vacuum conditions and require high operation costs. The particles provided by spray pyrolysis are too large to be useful. Gel method can provide a desired particle size but requires high costs because it should start with metal alkoxides which are expensive and easily flammable. Hydrothermal method is a modification of the gel method and has to use a hydrothermal apparatus which should be conducted under high temperature and high pressure conditions. Hydrothermal method can start with metal salts without the shortcomings of the gel method. However, hydrothermal method has the disadvantage of high operation costs for the high temperature and pressure conditions of the hydrothermal apparatus.
Therefore, there is a necessity in the art to provide an economical process for preparing ultra-fine metal oxide powders with desired particle size and particle distribution without the utilization of vacuum or high pressure condition.